Vendor Branches

As is especially the case when developing software, the data
that you maintain under version control is often closely related
to, or perhaps dependent upon, someone else's data. Generally,
the needs of your project will dictate that you stay as
up to date as possible with the data provided by that external
entity without sacrificing the stability of your own project.
This scenario plays itself out all the time—anywhere that
the information generated by one group of people has a direct
effect on that which is generated by another group.

For example, software developers might be working on an
application that makes use of a third-party library. Subversion
has just such a relationship with the Apache Portable Runtime (APR)
library (see the section called “The Apache Portable Runtime Library”).
The Subversion source code depends on the APR library for all
its portability needs. In earlier stages of Subversion's
development, the project closely tracked APR's changing API,
always sticking to the “bleeding edge” of the
library's code churn. Now that both APR and Subversion have
matured, Subversion attempts to synchronize with APR's library
API only at well-tested, stable release points.

Now, if your project depends on someone else's information,
you could attempt to synchronize that information with your own
in several ways. Most painfully, you could issue oral or
written instructions to all the contributors of your project,
telling them to make sure they have the specific versions
of that third-party information that your project needs. If the
third-party information is maintained in a Subversion
repository, you could also use Subversion's externals
definitions to effectively “pin down” specific
versions of that information to some location in your own
working copy directory (see
the section called “Externals Definitions”).

But sometimes you want to maintain custom modifications to
third-party code in your own version control system. Returning
to the software development example, programmers might need to
make modifications to that third-party library for their own
purposes. These modifications might include new functionality
or bug fixes, maintained internally only until they become part
of an official release of the third-party library. Or the
changes might never be relayed back to the library maintainers,
existing solely as custom tweaks to make the library further
suit the needs of the software developers.

Now you face an interesting situation. Your project could
house its custom modifications to the third-party data in some
disjointed fashion, such as using patch files or full-fledged
alternative versions of files and directories. But these quickly
become maintenance headaches, requiring some mechanism by which
to apply your custom changes to the third-party code and
necessitating regeneration of those changes with each successive
version of the third-party code that you track.

The solution to this problem is to use vendor
branches. A vendor branch is a directory tree in
your own version control system that contains information
provided by a third-party entity, or vendor. Each version of
the vendor's data that you decide to absorb into your project is
called a vendor drop.

Vendor branches provide two benefits. First, by storing
the currently supported vendor drop in your own version control
system, you ensure that the members of your project never need to question
whether they have the right version of the vendor's data. They
simply receive that correct version as part of their regular
working copy updates. Second, because the data lives in your
own Subversion repository, you can store your custom changes to
it in-place—you have no more need of an automated (or
worse, manual) method for swapping in your customizations.

General Vendor Branch Management Procedure

Managing vendor branches generally works like this: first,
you create a top-level directory (such as
/vendor) to hold the vendor branches.
Then you import the third-party code into a subdirectory of
that top-level directory. You then copy that subdirectory
into your main development branch (e.g.,
/trunk) at the appropriate location. You
always make your local changes in the main development branch.
With each new release of the code you are tracking, you bring
it into the vendor branch and merge the changes into
/trunk, resolving whatever conflicts
occur between your local changes and the upstream
changes.

An example will help to clarify this algorithm. We'll use
a scenario where your development team is creating a
calculator program that links against a third-party complex
number arithmetic library, libcomplex. We'll begin with the
initial creation of the vendor branch and the import of the
first vendor drop. We'll call our vendor branch directory
libcomplex, and our code drops will go
into a subdirectory of our vendor branch called
current. And since svn
import creates all the intermediate parent
directories it needs, we can actually accomplish both of these
steps with a single command:

We now have the current version of the libcomplex source
code in /vendor/libcomplex/current. Now,
we tag that version (see the section called “Tags”)
and then copy it into the main development branch. Our copy
will create a new directory called
libcomplex in our existing
calc project directory. It is in this
copied version of the vendor data that we will make our
customizations:

We check out our project's main branch—which now
includes a copy of the first vendor drop—and we get to
work customizing the libcomplex code. Before we know it, our
modified version of libcomplex is now completely integrated
into our calculator program.
[26]

A few weeks later, the developers of libcomplex release a
new version of their library—version 1.1—which
contains some features and functionality that we really want.
We'd like to upgrade to this new version, but without losing
the customizations we made to the existing version. What we
essentially would like to do is to replace our current
baseline version of libcomplex 1.0 with a copy of libcomplex
1.1, and then re-apply the custom modifications we previously
made to that library to the new version. But we actually
approach the problem from the other direction, applying the
changes made to libcomplex between versions 1.0 and 1.1 to our
modified copy of it.

To perform this upgrade, we check out a copy of our vendor
branch and replace the code in the
current directory with the new libcomplex
1.1 source code. We quite literally copy new files on top of
existing files, perhaps exploding the libcomplex 1.1 release
tarball atop our existing files and directories. The goal
here is to make our current directory
contain only the libcomplex 1.1 code and to ensure that all
that code is under version control. Oh, and we want to do
this with as little version control history disturbance as
possible.

After replacing the 1.0 code with 1.1 code, svn
status will show files with local modifications as
well as, perhaps, some unversioned files. If we did what we
were supposed to do, the unversioned files are only those new
files introduced in the 1.1 release of libcomplex—we
run svn add on those to get them under
version control. If the 1.1 code no longer has certain files
that were in the 1.0 tree, it may be hard to notice them;
you'd have to compare the two trees with some external tool
and then svn delete any files present in
1.0 but not in 1.1. (Although it might also be just fine to
let these same files live on in unused obscurity!) Finally,
once our current working copy contains
only the libcomplex 1.1 code, we commit the changes we made to
get it looking that way.

Our current branch now contains the
new vendor drop. We tag the new version as 1.1 (in the same
way we previously tagged the version 1.0 vendor drop), and
then merge the differences between the tag of the previous
version and the new current version into our main development
branch:

In the trivial use case, the new version of our
third-party tool would look, from a files-and-directories
point of view, just like the previous version. None of the
libcomplex source files would have been deleted, renamed, or
moved to different locations—the new version would
contain only textual modifications against the previous one.
In a perfect world, our modifications would apply cleanly to
the new version of the library, with absolutely no
complications or conflicts.

But things aren't always that simple, and in fact it is
quite common for source files to get moved around between
releases of software. This complicates the process of
ensuring that our modifications are still valid for the new
version of code, and things can quickly degrade into a
situation where we have to manually re-create our
customizations in the new version. Once Subversion knows
about the history of a given source file—including all
its previous locations—the process of merging in the new
version of the library is pretty simple. But we are
responsible for telling Subversion how the source file layout
changed from vendor drop to vendor drop.

svn_load_dirs.pl

Vendor drops that contain more than a few deletes,
additions, and moves complicate the process of upgrading to
each successive version of the third-party data. So
Subversion supplies the svn_load_dirs.pl
script to assist with this process. This script automates the
importing steps we mentioned in the general vendor branch
management procedure to make sure mistakes are minimized.
You will still be responsible for using the merge commands to
merge the new versions of the third-party data into your main
development branch, but svn_load_dirs.pl
can help you more quickly and easily arrive at that
stage.

In short, svn_load_dirs.pl is an
enhancement to svn import that has several
important characteristics:

It can be run at any point in time to bring an existing
directory in the repository to exactly match an external
directory, performing all the necessary adds and deletes,
and optionally performing moves, too.

It takes care of complicated series of operations between
which Subversion requires an intermediate commit—such
as before renaming a file or directory twice.

It will optionally tag the newly imported directory.

It will optionally add arbitrary properties to files and
directories that match a regular expression.

svn_load_dirs.pl takes three mandatory
arguments. The first argument is the URL to the base
Subversion directory to work in. This argument is followed by
the URL—relative to the first argument—into which the
current vendor drop will be imported. Finally, the third
argument is the local directory to import. Using our previous
example, a typical run of svn_load_dirs.pl
might look like this:

You can indicate that you'd like
svn_load_dirs.pl to tag the new vendor drop
by passing the -t command-line option and
specifying a tag name. This tag is another URL relative to
the first program argument.

When you run svn_load_dirs.pl, it
examines the contents of your existing “current”
vendor drop and compares them with the proposed new vendor
drop. In the trivial case, no files will be in
one version and not the other, and the script will perform the
new import without incident. If, however, there are
discrepancies in the file layouts between versions,
svn_load_dirs.pl will ask you how
to resolve those differences. For example, you
will have the opportunity to tell the script that you know
that the file math.c in version 1.0 of
libcomplex was renamed to arithmetic.c in
libcomplex 1.1. Any discrepancies not explained by moves
are treated as regular additions and deletions.

The script also accepts a separate configuration file for
setting properties on files and directories matching a regular
expression that are added to the
repository. This configuration file is specified to
svn_load_dirs.pl using the
-p command-line option. Each line of the
configuration file is a whitespace-delimited set of two or
four values: a Perl-style regular expression against which to match the
added path, a control keyword (either
break or cont), and then
optionally a property name and value.

For each added path, the configured property changes whose
regular expression matches the path are applied in order,
unless the control specification is break
(which means that no more property changes should be applied
to that path). If the control specification is
cont—an abbreviation for
continue—matching will continue
with the next line of the configuration file.

Any whitespace in the regular expression, property name,
or property value must be surrounded by either single or
double quotes. You can escape quotes that
are not used for wrapping whitespace by preceding them with a
backslash (\) character. The backslash
escapes only quotes when parsing the configuration file, so do
not protect any other characters beyond what is necessary for
the regular expression.

You are reading Version Control with Subversion (for Subversion 1.6), by Ben Collins-Sussman, Brian W. Fitzpatrick, and C. Michael Pilato.
This work is licensed under the Creative Commons Attribution License v2.0.
To submit comments, corrections, or other contributions to the text, please visit http://www.svnbook.com/.